This research proposal has two objectives: 1) To study the packaging and arrangement of SV40 DNA in virus particles as a model for the higher order organization of cell chromatin; and 2) to study the relationship between the structures and biological activities of SV40 nucleoprotein complexes. Preliminary studies using psoralen derivatives to cross-link SV40 DNA in vivo or in situ in the virus particle have detected novel DNA-protein interactions of DNA in the virus particle and in SV40 replicaton complexes. Experiments are proposed to extend these analyses.